1. The Field of the Invention
The present invention relates to a tool to be used as a component in a drill string and, in particular, to a tool having axial thrust bearings, capable of withstanding compressive loads; radial bearings, to insure that torque is not transmitted across the sub when in the rotary configuration; and means to selectively permit transmission of torque across the sub.
2. Background of the Invention
When non-buckled pipe is moved in the axial direction along a straight inclined well bore, there is an axial friction force that opposes movement of the pipe. This friction force is referred to as "gravity drag". The magnitude of this drag is determined by multiplying the normal force acting against the hole wall by an axial coefficient of friction. This normal force is the weight of the pipe multiplied by the sine of the hole inclination. In other words, it is the normal component of gravity. The higher the hole inclination, the higher the normal force.
When pipe is moved in the axial direction along a curved section of the well bore, there is an additional axial frictional force that must be considered. A curved section of a well bore is commonly referred to as a "dogleg". When pipe subjected to an axial load (either tensile or compressive), is in a section of the well bore where a dogleg exists, there is an additional normal force acting against the hole wall. This will be referred as a "dogleg normal force". The magnitude of the dogleg normal force is proportional to the magnitude of the pipe axial load (force). There is an axial frictional force associated with this dogleg normal force when the pipe is moving in the axial direction. This additional force, or dogleg frictional force, is determined by multiplying the dogleg normal force by the axial coefficient of friction.
When a compressive load on an element of pipe exceeds a critical buckling load, the pipe buckles. This buckling action produces a force normal to the hole wall. The magnitude of the buckling normal force is proportional to the square of the axial force acting on the pipe. There is also an axial frictional force associated with this buckling normal force when the pipe is moving in the axial direction. This buckling frictional force is determined by multiplying the buckling normal force by the axial coefficient of friction.
The buckling frictional force is often overlooked. This force can be substantial in the vertical section of the hole where the critical buckling loads are minimal. It is actually possible for the buckling frictional force to equal the gravity axial force. When this occurs, it is no longer possible to transmit weight downhole. In effect, the pipe in the hole becomes locked. Slacking off additional pipe weight will only result in the pipe stacking up above the section where the pipe is locked due to the magnitude of the buckling frictional force.
In a curved well bore, the total frictional force opposing the movement of pipe is determined by adding the gravitational frictional force to the dogleg frictional force and the buckling frictional force. It is this total frictional force that is commonly referred to as "drag". All three of these individual frictional forces (gravity, dogleg and buckling), are directly proportional to the axial coefficient of friction. Thus the drag is directly proportional to the axial coefficient of friction.
When lowering pipe into a well bore, the total drag force described above works in the opposite direction of the axial component of gravity. This axial component of gravity is referred to as the "axial gravity force". The axial gravity force is the weight of the pipe multiplied by the cosine of the hole inclination. The higher the hole inclination, the lower the axial component of gravity.
A critical angle exists where the total drag force is equal to the axial gravity force. Pipe being lowered into a well bore will not slide down the hole under its own weight if it is in a section of the well bore where the inclination is greater than the critical angle. A compressive force is required to push the pipe into sections of the well bore where the inclination is above the critical angle. The critical angle is a function of the axial coefficient of friction.
When non-buckled pipe is being lowered into a straight inclined well bore, the total drag force is equivalent to the gravity drag force. In this situation the critical hole angle is plus or minus 72.degree., assuming an average axial coefficient of friction of 0.33. In this case, all pipe lowered into the hole at inclinations above 72.degree. would have to be pushed into the hole. With a higher friction factor, the critical hole angle would be less than 72.degree..
If pipe is being lowered into a curved well bore, the critical hole angle is even less than that of a straight well bore, due to the dogleg frictional force. The higher the axial load on the pipe, the higher the dogleg frictional force and the lower the critical angle. In horizontal wells with long horizontal sections and/or in high angle extended reach wells, it is possible for the critical angle to actually be in the vertical section of the wellbore (at 0.degree. inclination).
In order to lower pipe into the well, the axial gravity force associated with the pipe at inclinations less than the critical angle must exceed the drag forces associated with the pipe at inclinations above the critical angle. In directional wells, axial drag forces increase as the horizontal displacement from the drilling location increases. Horizontal displacement can be significantly limited in shallow horizontal and/or extended reach wells, since the axial gravity force available to push pipe into the hole is limited by the depth of the well.
The maximum achievable horizontal displacement in extended reach and/or horizontal wells can be increased by lowering the drag (axial component of friction). This can be done by decreasing the axial coefficient of friction. The axial coefficient of friction may be reduced by spotting a lubricating agent in the wellbore. Another means of reducing the axial coefficient of friction would be by rotating the pipe in the hole. This latter means is an effective solution of the problem, since the axial coefficient of friction is minimal as long as the pipe is rotating in the hole.
However, sometimes it is not desirable to rotate certain components of a drillstring. These components are typically located in the lower section of the drillstring. For example, a mud motor cannot be rotated during a course correction run or the tool face orientation will be lost. It is also often undesirable to rotate the drillstring when running a liner in an extended reach well, since the torsional stress might exceed the torsional yield of the connections on the liner. The axial drag could still be reduced in these situations, if a means existed to rotate the upper section of the drillstring without rotating the lower section of the drillstring. The present invention addresses and provides a unique solution for this problem.
The subject downhole rotary bearing sub includes means for rotating the pipe above the sub without rotating the pipe below the sub. By rotating the pipe above the sub, the axial coefficient of friction acting upon this upper section of pipe will be minimal and thus the axial drag forces will also be minimal. The pipe below the sub will not be rotated while being run in the hole and will not be subject to torsional loads. This pipe, however, will be subject to normal axial frictional forces.